1. Field of the Invention
This invention relates to a measuring device, more particularly to a chain force measuring device adapted for mounting on a bicycle.
2. Description of the Related Art
A conventional chain force measuring device is generally used to measure a force or torque exerted by a cyclist so as to control a driving force supplied by an electric drive motor of an electrically supported bicycle. As described in international patent applications WO 01/30643 and WO 03/073057, a force measuring device has strain gauges to detect a bending of a rear axle of a bicycle when a cyclist is pedaling to obtain a measurement signal of a chain force. A controller of the bicycle receives the measurement signal of the chain force to drive an electric drive motor of the bicycle so as to provide a proportional driving force. Therefore, the cyclist needs not to apply too much force on pedals.
Referring to FIG. 1, U.S. Pat. No. 7,814,800 B2 discloses another chain force measuring device which utilizes a hall sensing unit to measure the chain force and which is mounted on a dropout member 90 of an electrically supported bicycle. The hall sensing unit includes a hall sensor 95 and a magnetic element 94. The dropout member 90 includes a frame connecting part 91, an axle connecting part 92, and a rigidity-reduced part 93. The frame connecting part 91 is connected to a frame fork of the bicycle. The axle connecting part 92 is formed with an elongated slot 921 permitting the rear axle to extend therethrough. The rigidity-reduced part 93 includes a lip portion 911 which is movable along a front-rear direction that is transverse to the rear axle when the chain force is exerted on the rear axle, and which is mounted with the magnetic element 94. The hall sensor 95 is disposed on the frame connecting part 91 that is spaced apart from and corresponds in position to the magnetic element 94.
When a cyclist is pedaling, the chain force exerting on the rear axle makes the rigidity-reduced part 93 deform and move relative to the frame connecting part 91 in the front-rear direction. At the same time, the lip portion 911 moves relative to the hall sensor 95 together with the magnetic element 94 in the front-rear direction and a magnetic field generated by the magnetic element 94 is changed so as to give off a signal that is indicative of the magnitude of the chain force and that is received by the hall sensor 95.
However, the hall sensor 95 is disposed to be spaced apart from the magnetic element 94 without a spacer. Consequently, when the rigidity-reduced part 93 deforms, the magnetic element 94 may be in contact with the hall sensor 95, thus adversely affecting measuring accuracy of the chain force measuring device.